The present invention relates to an adaptive equalizer arrangement for digital transmission systems, and more specifically to high-speed systems in which the transmission channel is not known in advance and/or is susceptible to variations with time. It is therefore suitable for use in digital radio links, data transmission over the switched telephone network, digital transmission over cables (special networks of the Traspac type, etc.).
The use of adaptive equalizers in high-speed digital transmission systems to compensate for the amplitude and phase distortions of the channel has been common practice during several years already. After their introduction in systems for data transmission over the switched telephone network the adaptive equalizers will be used in digital radio links in the near future. The equalizers used in practice have generally one of the following structures: (a) a non-recursive transversal filter, (b) a transversal filter having a recursive portion whose input is constituted by the previously decided symbols. Adapting the equalizer to the channel and to its possible variations with time is generally effected with the aid of a method known as the stochastic gradient of the mean-square error. The two equalizer structures mentioned above and their adaption have been described in several articles, inter alia in: C. Macchi et al., "Recepteurs adaptatifs pour transmission de donnees a grande vitesse", Annales des Telecommunications, Vol. 30, No. 9-10, September-October 1975.
The equalizers often operate in the baseband and consequently act on demodulated signals. On the other hand, the systems of a high spectral efficiency employ modulation of two quadrature carriers. In these systems, to compensate for the intersymbol interference in the in-phase and quadrature-phase paths and to compensate for the interference between these two paths, the equalizer must have four branches, each of which is constituted by a transversal filter; the recursive portion, if any, likewise comprises four transversal filters.
Another disadvantage of baseband equalizers will become apparent in considering the synchronization of the carrier necessary for the demodulation. Actually, the recovery of the carrier in systems employing modulation of two quadrature carriers utilizes the baseband signals and the decisions (see the article by A. Leclert and P. Vandamme, published in IEEE Transactions on Communications, Vol. COM-31, No. 1, January 1983, pages 130 to 136). There are therefore two possibilities: (a) either the signals at the output of the demodulator and the decisions at this point are used, in such a case the system is not very robust as it is very sensitive to channel distortions, (b) or the two output signals of the equalizer and their decisions are used, in such a case the system is not sensitive to channel distortions. However, in the latter case, the carrier recovery loop comprises an additional delay which is the propagation time of the signals through the equalizer and which, when the equalizer is very long (as is the case in data transmission over cables), tends to render the carrier recovery loop unstable. The loop will then not be capable of tracking large frequency deviations (as is described in, for example, the article by R. W. Chang and R. Srinivasagopalan, "Carrier recovery for data communication systems with adaptive equalization" published in IEEE Transactions on Communications, Vol. COM-28, No. 8, August 1980, pages 1142 to 1153). The same problem occurs in digital equalizers, even if they do not comprise many coefficients, as the propagation time cannot be disregarded.
With regard to this delay problem in baseband equalizers, intermediate-frequency equalizers, denoted IF-equalizers, have been introduced (D. D. Falconer, "Jointly Adaptive Equalization and Carrier Recovery in Two-Dimensional Digital Communication Systems", BSTJ, Vol. 55, No. 3, March 1976, pages 317-334). In the systems using IF-equalization, the carrier recovery is very robust because of the double advantage that equalized signals are used and the delay of the equalizer plays no part in the carrier recovery. The IF-equalizer of the Falconer type, as do also the baseband equalizers, utilizes the mean-square error criteron for the adaption. Its disadvantage is that it requires sampling of the IF-signal at the symbol rate and, for its adaption, the remodulation of decisions of the equalizer. The need for sampling of these two IF-signals may cause problems for a practical implementation, as the IF-carrier frequency is high. The sampling of the IF-signal is then very sensitive to jitter at the sampling instant. In any case, it is more difficult to sample an IF-signal than to sample an associated baseband signal.